We are frequently told that America should be ashamed of having sent so many people to prison. We are compared unfavorably to most of Europe. But these complaints rarely ask what benefits flow from prison.

The best scholars have estimated that between 25 and 30 percent of the recent decline in crime rates is the result of imprisonment. A comparison with England is helpful. At one time it imprisoned a higher fraction of offenders than did the US, but in the 1980s it changed by imprisoning fewer people. As a result (I think), the British crime rate soared while ours fell.

Between 1980 and 1985 the American prison population increased by more than half and between 1985 and 1990 it again increased by half. But from 1987 to 1992, the British prison population dropped by about five thousand inmates despite a sharp rise in the crime rate.

These different responses did not happen by accident. Americans, voting for district attorneys, mayors, and governors, chose people who would take crime seriously. In England hardly any of these offices are filled by local election; instead, the Parliament and the Home Office decide on crime policies.

Those decisions included a bill that urged judges not to send offenders to prison unless the crime was very serious, and in determining seriousness the judges were asked to ignore the prior record of the offenders.

In short, American policies were driven by public opinion while British ones were shaped by elite preferences. As a result, victim surveys show that by the late 1990s the British robbery rate was one-quarter higher and the burglary and assault rates twice as high as those in this country.

This raises the interesting question of why elite views should be so different from popular ones. Some possible explanations: Elites can more easily protect themselves from criminal attacks; elites tend to have a therapeutic rather than punitive view of crime; elites in parliamentary regimes are protected against sharp swings in public moods.

There are a lot of criticisms one can make of prisons, but sending offenders there, provided it is done correctly and without abuse, is an eminently democratic strategy: We deprive guilty people of liberty to make innocent people safer.

Several readers have asked whether prison deters crime, some saying that correlation is not the same as causation. Social scientists have known that dictum for the better part of a century and have worked hard at finding out whether prison deters crime among would-be-offenders (it obviously prevents it among people already locked up). Steven Levitt, principal author of Freakonomics, discusses the issue in that book and summarizes the evidence, now very strong, in the chapter he wrote from a book I and Joan Petersilia edited entitled Crime. The econometric technique is simple: construct an equation that asks what can explain the crime rate using, as explanations, everything that we think causes crime (for example, urbanization, unemployment, and the like) and adds the chances of going to prison. When you do this, you find that the higher the chances of going to prison in a state, the lower the crime rate, other things being equal. (You also discover that the unemployment rate has very little effect on crime.)

Some readers have asked whether the population of American prisons is large because we lock up so many drug users. It is true that the proportion of inmates described as drug offenders has gone up dramatically, but as Jonathan Caulkins and Mark Kleiman point in their essay in Understanding America, very few are in prison because of drug possession. Many are either major dealers or plead down to a drug possession charge in order to avoid being convicted of a more serious offense. There are more than one million arrests every year for drug possession, but very of them result in prison or jail time. Cannabis possession, when it is punished at all, is typically with a fine or probation.

A lot of readers have suggested that I am not a helpful blogger because I refer people to other studies for data to support my arguments. These critics are probably right. Were I devoted to blogging full time, I would quote all the data and summarize all of the studies, thereby getting nothing else done. I had assumed when I started my blog messages that people would pause, think, and look up facts. A few have, but most seem to have opinions they like to express quickly. There is nothing wrong with this, except that it doesn't advance knowledge. Let me join the opinion parade by offering a few of my own: This country imprisons too many people on drug charges with little observable effect. A better solution can be found in Hawaii, where a judge uses his powers to keep drug users in treatment programs (it's called Project Hope; look it up). The costs of crime are hard to measure (so are the costs of confinement). The reader who does not want to drive five miles to find the book, Prison State, that discusses this in detail is wasting my time and his. It is not hard to study deterring crime, but I can't imagine trying to teach someone in a blog how to do a regression analysis. I wish I could do that, but it would take time, and blog commenters seem not to have much time.

Now for a few more facts, but I warn you that to believe my assertions you will actually have to go out and read something. Intensive Probation: This is a good idea, but so far the studies of it have not suggested it lowers the crime rate. I wish it did, because it is cheaper than prison. The chief study, done at RAND, compared probationers under intensive supervision with similar ones not under such control. There as no difference in their crime rates while under supervision. There are two possible explanations for this: Either there was no difference in crime rates, or those under intensive supervision had more crimes noticed by their probation officers.

The effect of prison on crime rates: The chief study is by William Spelman and appears in the book, The Crime Drop in America, edited by Alfred Blumstein and Joel Wallman (2000).

The problem of the mentally ill in prison: This is a very large problem, caused in part by the dramatic drop in the number of mentally ill people in mental institutions. De-institutionalizing the mentally ill may have been a good idea, but the price we pay for it is to have a big increased of the mentally ill in prison. I would prefer that they be in mental health care institutions, but our society does not let that happen.

Abortion and crime rates: Steven Levitt is one of the authors of the study claiming, on the basis of a study of crime rates in five states where abortion was legal before Roe v. Wade and the 47 states where it became legal after Roe v. Wade, that abortion reduced the number of "unwanted children" and hence helped lower the crime rate. Levitt may be right, but to show that it is first necessary to count abortions in many states where, though it was illegal before Roe, occurred anyway to protect the health of the mother, and to control for other factors that affect the crime besides the number of "unwanted" children.